OBJECTIVE. To measure sexual behavior changes (especially the use of condoms) among adolescents after 5 years of a population-based acquired immunodeficiency syndrome prevention strategy at the national and regional level. DESIGN. Two comparative cross-sectional surveys by self-administered questionnaires were conducted in 1987 and 1990. SETTING. Two similar samples of 16- to 19-year-old apprentices. The two samples, 1987 (n = 1359) and 1990 (n = 817), are comparable in terms of sex, age, profession, and location. RESULTS. The percentage of boys who say they are sexually active has remained the same (75%) and has slightly increased among younger girls (65% to 75%, NS). The average number of partners has remained steady for boys and girls. Regular use of any contraceptive device has remained steady among girls (75%) and increased among boys (1987, 38%; 1990, 54%; P < .001). In 1987 as well as in 1990, the condom was the most widely used method (> 90%), followed by the pill (50%), withdrawal (20%), and chemicals (5%). Rates were about the same for both sexes. Rates of irregular use of condoms increased among boys and girls (P < .01); regular use of condoms increased among boys (22% to 34%, P < .01) and girls (10% to 27%, P < .001); use of the pill has slightly decreased. CONCLUSIONS. During a 5-year period, the Swiss "STOP-AIDS" campaign and local interventions had had no real effect on the rate of sexual activity of apprentices but had had a positive effect on the use of contraception and condoms. The prevention strategy should (and will) be maintained in the future.